Series-transformer circuit



June 5, 1962 CURRENT CORE FLUX F1 gt. 5

SECONDARY VOUZIGE W/THOUT F/LTER SECONDARY v01 7716f A. KUSKO SERIES-TRANSFORMER CIRCUIT Filed April 1, 1957 T/ME INVENTOR ALEXA/V051? KUSKO ATTORNEY g Mai United States Patent 3,038,113 SERIES-TRANSFORMER CIRCUIT Alexander Kusko, Cambridge, Mass, assignor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed Apr. 1, 1957, Ser. No. 649,967 3 Claims. (Cl. 323--61) This invention relates to electrical transformers and circuits for operating constant-voltage apparatus from constant-current alternating-current electrical lines. The invention is especially applicable to the operation of lighting equipment from constant-current lines, such operation being useful in the lighting of streets and of airport landing strips.

An object of the invention is to produce a device through which constant-voltage apparatus can be connected to a constant-current line. An especial object of the invention is to produce such a device with good output voltage regulation over a considerable range of current input. The latter is especially important in the operation of electronic flash approach systems for airports, because the current in the constant-current circuit is varied to change the intensity of the runway and other lights, and the voltage across the electronic flash approach system should remain constant despite such variations.

I have discovered that such results can be achieved by the use of a saturable-core transformer between the constant-current line and the equipment to be operated. Such a transformer would of itself produce a voltage output of poor waveform; in fact, the Waveform is found to be a series of well-spaced spikes, and is unsuitable for many purposes.

However, I have further discovered that by connecting a filter across the transformer, I can improve the Waveform to a much closer approximation to a sinusoid. The filter can be a series resonant circuit tuned to the third harmonic, which is especialy strong in a waveform of the type described.

From the standpoint of the load circuit the filter can be considered as short-circuiting the third harmonic voltage and removing its effect on the load. From the standpoint of the transformer, however, the filter can be assumed to pass just enough circulating current to cancel out the third harmonic in the core flux wave and to thereby prevent the third harmonic from even being induced in the voltage wave.

Additional filters, tuned to other harmonics, can be used if desired, but generally a single filter to remove the third harmonic will be sufficient. If the filter is tuned to a frequency somewhat above the third harmonic, that is to a frequency between that of the third and fifth harmonics, the impedance to the third harmonic can be kept small enough for filtering that frequency, while the impedance for the fifth and higher harmonics is reduced enough to improve the filtering for them.

I have found that the use of a saturable-core transformer with a single filter to remove the third harmonic will maintain the secondary voltage within of a mean value despite variations of more than 200% in the input current.

Moreover, the foregoing results are achieved with no moving parts, with components whose life should be about equal to that of the customary life of a transformer, and with no need for adjustment or field maintenance after installation.

It will be understood, of course, that when I refer to constant-current circuits, I means a series-type circuit in which the current is held reasonably constant at a given value, which may, however, be varied from one given value to another given value if desired, for example, to

effect a variation in intensity of lamps operating from it. By constant-voltage, I means a source whose voltage remains constant within reasonable limits. For example in using my device in certain airport lighting circuits, the current in the primary could be held constant at various values from 8.5 amperes to 20 amperes, while the voltage output of my device varied only from about 230 volts to about 250 volts.

Other objects, features and advantages of my invention will be apparent from the following description, taken in conjunction with the accompanying drawings in which:

PEG. 1 shows a circuit diagram of a device according to the invention;

FIG. 2 is a graph of the waveform of the input current to the primary of the transformer of my device;

FIG. 3 is a graph of the waveform of the flux resulting from that input current flowing through the turns of the primary;

FIG. 4- is a graph of the waveform of the output voltage of the transformer secondary without a filter; and

PEG. 5 is a graph of the waveform of the output voltage across the transformer with the filter in place.

In FIG. 1, the primary 1 of transformer 2 is electrically connected in series with the constant-current line 3, 3. The current in said line is held substantially constant, despite variations in load, by well known types of equipment customary for that purpose. In order to keep the power factor of the line high, and preferably at about unity, the condenser 4 is used in series with the primary 1. Otherwise, the magnetizing volt-amperes for the primary 1 would produce a large reactive volt-ampere component in the series circuit, which would reduce the output obtainable from the constant-current regulator and if the regulator is of the resonant type using a condenser, might increase the voltage across that condenser above the breakdown value.

The primary 1 is wound on core 5, upon which is also wound the secondary winding 6. A filter 7, comprising a condenser 8 and inductance coil 9 in series, is connected across the secondary 6, and will be in shunt to the load terminals 10, 11 of the secondary 6.

In one embodiment, in which the electrical power line 3, 3 could be set for any of a variety of constant-currents from 8.5 to 20 amperes at a frequency of about 60 cycles per second, the primary 1 of transformer 2 comprises 55 turns of copperfoil 0.006 inch in thickness and 2% inches in width, wound in a single-turn spiral, with a sheet of paper between the turns, for example as shown in the application Serial No. 401,333, filed December 30, 1953, by Albert Zack. The primary 1 fitted over, and was insulated from, a core of laminations of the socalled dynamo special steel, that is, a steel containing 4% silicon by weight, each lamination being of 24-gauge 1% inches wide, tand'being stacked up to a height of 2% inches. The coil was on the center leg of a standard E 1 configuration, 7 inches by 4% inches in outside dimensions.

The secondary winding 6, wound over primary 1 and insulated therefrom, comprises 247 turns of enamelled #17, B. & S. gauge, copper wire. The core material of the transformer was such that approximate saturation was reached at a primary current of about six and onehalf amperes, so that the core would be well saturated at the lowest current for which it was designed, namely 8.5 amperes.

The condenser 8 was of 30 microfarads, and could be from about 28 to 35 microfar-ads for best results, with an inductance coil 9 of about 29 millihenries in conjunction with the transformer described.

The inductance coil used had 300 turns of enameled #16 copper wire, B. & S. gauge, wound on the center leg of a standard E-I core of dynamo special steel, that Patented June 5, 1962 is a steel containing 4% silicon. The outside dimensions of the EI core were about 3 inches by 4 inches, with the laminations being 1% inch wide and stacked to a height of 2 /2 inches, that is the core cross-section was 1% by 2 /2 inches on the center leg, the otherlegs being, of course, only half as wide. The ES and the PS of the core were butted together in the usual manner with an air gap of 0.028 inch in each of the three legs, giving an effective air gap of 0.056 inch. Fibre spacers were used in the gap to provide the proper spacing and the unit assembled in the usual manner, Well-known in the art.

If the coil 9 were of different inductance, the condenser would be changed correspondingly for best results. A larger inductance would require a smaller condenser and vice versa; however, the value of inductance and capacitance should be such as to provide, at the third harmonic of the primary input, an impedance small compared to the impedance of the load at the third harmonic.

When the inductance and capacitance of the filter are in resonance at the third harmonic frequency, the filter impedance at that frequency will be merely equal to the combined series resistance of the condenser and inductance. The filter can, however, be detuned, that is, it can be resonant at a frequency somewhat above the third harmonic, if its impedance to the third harmonic is nonetheless small compared to that of the load. Operation under such a condition, with a resonant frequency somewhat above the third harmonic but below the fifth harmonic, will have the advantage of suppressing not only the third harmonic in the voltage, but also of considerably reducing the fifth and higher harmonics.

In FIG. 2, which is a plot of voltage against time for several cycles of the alternating current input, the sinusoidal waveform of the input current is seen. Because of the saturation of the core, the waveform of the flux produced by the current will not be sinusoidal, but will have a fiat top because the flux cannot rise above a saturation value. The plot of flux against time is shown in FIG. 3.

Since voltage is only induced when the flux is changing, there will be no voltage during the time interval corresponding to the flat top of the flux curve, and so the volt-age wave will have the form of a spaced spike as in FIG. 4. If a grain-oriented steel, such as a ferromagnetic alloy having a sharp saturation characteristic, for example, an alloy of 50% nickel and 50% iron by weight, is used as the core material, the area under the voltage spike will be constant for various input currents, the height of the spike decreasing and the width increas ing at low currents. The average half-cycle voltage will thus remain constant. With a silicon steel core such as that used in the specific example described above, the area under the spike will vary somewhat, and the average volt-age will change slightly, but the regulation will still be very good, as explained above.

When the filter is connected across the tnansformer output, the third harmonic is suppressed in the output voltage wave and the waveform becomes similar to that in FIG. 5, which is for the case where there is also some suppression of the fifth harmonic. If the fifth is not reduced, there will be some slight bumps in the top of the curve.

In the specific example described, the load on the secondary of the transformer was an electronic flash approach system lighting unit taking about 1.8 amperes. In addition, the filter took about 2 ampercs.

Although the invention has been described with respect to a specific embodiment, various modifications will be apparent to those skilled in the art without departing from the spirit of the invention.

What I claim is:

l. A converter for maintaining a substantially constant secondary voltage output over a wide range of primary current inputs, said converter comprising a saturable core transformer having a primary and secondary and a saturable ferromagnetic core linking the entire primary and the entire secondary, and a series circuit resonant to a frequency above the third harmonic of the primary current but below the fifth harmonic thereof connected across the tnansformer alternating current secondary.

2. A converter for maintaining a substantially constant secondary voltage output over a wide range of primary current inputs, said converter comprising a saturable core transformer having a primary and secondary and a saturable ferromagnetic core linking the entire primary and the entire alternating circuit secondary, and a series circuit resonant to a frequency above the third harmonic, but below the fifth harmonic of the input current conneoted across the transformer secondary, said saturable ferromagnetic core being designed to saturate at a current below a predetermined value corresponding to the minimum operating current of the transformer.

3. A voltage regulator comprising a saturable-core transformer, a filter passing the third harmonic connected across the alternating current output of the transformer, and -a condenser connected in series with the input to the tnansformer.

References Cited in the file of this patent UNITED STATES PATENTS 1,174,793 Alexanderson Mar. 7, 1916 1,749,841 Nyquist Mar. 11, 1930 1,819,299 Miller Aug. 18, 1931 1,860,543 Kouyoumjian May 31, 1932 1,870,851 Jones Aug. 9, 1932 2,338,080 Brown Dec. 28, 1943 2,444,715 Walker July 6, 1948 2,579,313 Gilson et al Dec. 18, 1951 2,763,827 Evans Sept. 18, 1956 2,764,725 Buie Sept. 25, 1956 

